This reverts commit 2c9fe4da.

ns4 is now on Debian Bullseye. Which means newer Docker and Systemd's
unified Cgroup hierarchy. At least I think that's why /proc/1/cpuset no
longer has the cgroup path.

No much point in just running pre-commit.

But only on the master branch and if pre-commit doesn't fail.

- Adding checks.
- Add to CI.
- Address issues.

Build on the webserver and save the files in the right directory.
Relax the Python version in Pipfile (I use a different version locally
to the one in CI). Set the language and locale to C.UTF-8 (the Docker
image doesn't come with other locales/ languages). Correct the submodule
URL to avoid the redirection warning.